This research is focused on the structure of proteins and amino acids that are postsynthetically modified by the Maillard reaction in vivo. These reactions may be significant in the aging process. Since the cross-linked, modified amino acids in these proteins cannot be readily volatilized for conventional GC/MS, our work depends heavily on FAB-MS, as well as on MS-MS for the characterization of the modified residues. Specific structures that have been identified in the past two years include lysine-pyrroles from L-threose, lysine crosslinks from glyoxal, hydroxymethylpyrroles from glucose and lysine, and glyoxal adducts of L-arginine.